switching

A little more detail needed as to the actual application.

Tom

You mean something like this? (view in fixed-width font, for example, with Notepad)

Batt 1 Batt 2

Michael

Michael, This is basically what I want to do. The battery will run a very small motor and a light.

Li

Looks like a double-pole-double-throw switch will work

see

seems simpler than a MOSFET solution.

When exactly do you want to switch?

Michael

This was one of the initial idea, Maybe have a servo motor to control the switch. ( and a PIC chip to supervise the whole operation) K

What about using a relay instead of a motor? Again I do not want too much energy loss while the relay is on

Li

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Relays will need way more power than a MOSFET.

Others will have to provide the MOSFET schematics. I have a hunch some P-channel MOSFETs may be needed, but I'm a beginner in this area (and would like to see the experts post their schematics ;-)

Michael

Use a relay - that's an electrically controlled switch.

There is probably no need to switch both positive and negative terminals - using an SPDT switch in the positive leads would likely be sufficient.

An output of a PIC probably won't drive a 5 amp relay directly, so you would have to have the PIC drive a transistor or small relay, which would drive the large relay.

would there be anything else beside a relay? that would require energy only when switching is required?

Li

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Try looking up a "latching relay". They only require power when they're switching. Driving a 5A load is going to require a realy with at least a 1 watt coil, so you'll need two transistors to drive the two coils from the PIC.

Cheers Chris

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Hi, That seems to be a good idea. I found this part SP4-L2-DC12V at digikey. But I have not seen any latching relay able to support 6 amp or something like 740watt (1 HP)

Li

Use a push-pull solenoid, tied to the end of a dpdt toggle switch...

It is a "do it yourself" latching relay with an arbitrary current carrying capability.

Regards, Bob Monsen

Another thought: Is there any reason you can't just connect the two batteries in parallel, and to the load. You will get the same total run time as you would using one battery, then switching to the other when the first is discharged.

In many applications, it is recommended that all available batteries should be connected and used/charged in parallel, rather than discharging part of the battery bank, then switching to the other part. This (use all in parallel) will generally result in a smaller depth-of-discharge of all the batteries, increasing their life (since depth-of-discharge is what usually kills rechargeable batteries.)

Actually, there are a number of relays that require well under 1 watt for the coil that can handle a 5A load. Digikey, Mouser, Newark - the usual suspects - all carry them. For example, Mouser part# 653-G2RL-24-DC12 is a DPDT relay with a 12V 33.33 mA (~.4 watts) coil, and has contacts rated 8 amps at 30 VDC. Even Allelectronics carries a few that handle 5 amps at the contacts and need well under a watt for the coil.

Ed